1. Field of the Invention
This invention relates to an analog comparator for detecting if an input signal voltage has reached a target or desired voltage as predetermined.
2. Description of the Prior Art
As an analog comparator of this type, there has been known one wherein a predetermined threshold voltage with the time delay of the circuit taken into account is set, it is detected that an input signal voltage having a certain slew rate has reached a voltage shifting by the predetermined threshold voltage from a target or desired voltage, and a coincidence timing signal representing that the input signal voltage has reached the target voltage is provided upon the detection.
In such an analog comparator, the threshold voltage has heretofore been fixed. In this case, a detection error in the time of arrival at the target voltage has developed in, for example, a use in which the slew rate, i.e., the slope of the input signal voltage changes sharply or a use in which the change of the threshold value ascribable to an ambient temperature change or the lapse of time becomes a problem.
More specifically, let it be supposed that an input signal voltage of comparatively low slew rate and an input signal voltage of comparatively high slew rate are respectively entered into the analog comparator having the predetermined threshold voltage with respect to the target voltage. In the former case, the input signal voltage reaches the target voltage in a comparatively long period of time t.sub.d1 after entering the threshold voltage. In the latter case, it reaches the target voltage in a comparatively short period of time t.sub.d2. Accordingly, in the case where the threshold voltage is set so that the period of time t.sub.d1 may become equal to a circuit delay time t.sub.d, the coincidence signal is correctly provided for the former, but the provision of the coincidence signal lags for the latter. Therefore, the detection error in the time of the arrival at the target voltage occurs.
Due to the change of the ambient temperature or the lapse of time, the threshold value changes, and the detection point fluctuates. Accordingly, the detection error in the time of the arrival at the target voltage takes place in a manner similar to that discussed above.